In This Article
- 1.Manufacturing Process
- 2.Mechanical Properties Comparison
- 3.Applicable Standards
- 4.When to Specify Forged
- 5.When to Specify Cast
Manufacturing Process
Cast valves are made by pouring molten metal into a mould — the shape solidifies as the metal cools. Casting allows complex shapes at low tooling cost and is economic for high volumes and large sizes. However, the cooling process can produce internal porosity, shrinkage cavities, and segregation that are not visible externally. Forged valves are made by pressing or hammering solid metal under high pressure into a die — the working process closes any voids, aligns the grain structure with the part geometry, and produces denser, stronger material with predictable mechanical properties. Forgings are more expensive for complex shapes but give superior mechanical properties.
Mechanical Properties Comparison
| Property | Forged (A105 CS) | Cast (WCB CS) | Impact |
|---|---|---|---|
| Tensile strength | 485 MPa min | 485 MPa min | Similar minimum spec |
| Yield strength | 250 MPa min | 250 MPa min | Similar minimum spec |
| Elongation | 22% min | 22% min | Similar minimum spec |
| Charpy impact (−46°C) | Superior grain structure | Lower toughness | Forged preferred for cryogenic |
| Internal porosity risk | Essentially zero | Requires RT/UT verification | Forged more reliable for critical service |
| Maximum size (economic) | Typically NPS 2" to 4" | No practical limit | Casting preferred above NPS 4"–6" |
| Weight (same rating) | ~30% heavier | Baseline | Forged heavier but fewer defects |
Applicable Standards
- Forged carbon steel: ASTM A105 (ambient to 450°C), ASTM A350 LF1/LF2 (low temperature service, impact tested to −46°C)
- Cast carbon steel: ASTM A216 WCB (ambient to 425°C), WCC (to 343°C, tougher grade)
- Forged stainless steel: ASTM A182 F304/F316/F316L
- Cast stainless steel: ASTM A351 CF8/CF8M (equivalent to 304/316)
- API 602: Small forged steel gate, globe, and check valves — NPS 4" and smaller. Specifically covers forged body valves.
- API 600: Cast steel gate valves — NPS 2" and larger. Specifically covers cast body gate valves.
When to Specify Forged
- NPS 2" and below — forgings are economical at small sizes and provide superior pressure-containing integrity
- Cryogenic service (below −46°C) — forged bodies have better impact toughness for low-temperature service; specify ASTM A350 LF2 with impact testing at −46°C
- High-pressure Class 800–2500 instrument and needle valves — forged body is standard
- Critical service where internal defects are unacceptable — forged eliminates porosity risk
- Socket-weld and threaded end connections — forged is the standard body form for small-bore SW and NPT valves
When to Specify Cast
- NPS 3" and above (cost advantage) — casting is more economical above NPS 3"
- Complex body geometries — Y-pattern globe valves, three-way valves, and other complex shapes are more economical in cast form
- API 600 gate valves — the standard specifically applies to cast body
- Large bore valves (NPS 6" to 48") — forgings at this size are extremely expensive and rarely justified unless the application demands it
- When porosity risk is managed — specify radiographic (RT) examination to ASME B16.34 Annex B for critical cast valves
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